FindBZip2: Populate BZIP2_INCLUDE_DIRS result variable

The plural-named non-cached result variable is needed to follow our
module conventions documented in `cmake-developer(7)`.  The variable is
also used to populate our ``BZip2::BZip2`` imported target include
directories, which was broken without this variable set.

Fixes: #18013

Modules/FindBZip2.cmake

@@ -20,14 +20,22 @@
 #
 # ``BZIP2_FOUND``
 #   system has BZip2
-# ``BZIP2_INCLUDE_DIR``
-#   the BZip2 include directory
+# ``BZIP2_INCLUDE_DIRS``
+#   the BZip2 include directories
 # ``BZIP2_LIBRARIES``
 #   Link these to use BZip2
 # ``BZIP2_NEED_PREFIX``
 #   this is set if the functions are prefixed with ``BZ2_``
 # ``BZIP2_VERSION_STRING``
 #   the version of BZip2 found
+#
+# Cache variables
+# ^^^^^^^^^^^^^^^
+#
+# The following cache variables may also be set:
+#
+# ``BZIP2_INCLUDE_DIR``
+#   the BZip2 include directory
 
 set(_BZIP2_PATHS PATHS
   "[HKEY_LOCAL_MACHINE\\SOFTWARE\\GnuWin32\\Bzip2;InstallPath]"
@@ -56,6 +64,7 @@ FIND_PACKAGE_HANDLE_STANDARD_ARGS(BZip2
                                   VERSION_VAR BZIP2_VERSION_STRING)
 
 if (BZIP2_FOUND)
+   set(BZIP2_INCLUDE_DIRS ${BZIP2_INCLUDE_DIR})
    include(${CMAKE_CURRENT_LIST_DIR}/CheckSymbolExists.cmake)
    include(${CMAKE_CURRENT_LIST_DIR}/CMakePushCheckState.cmake)
    cmake_push_check_state()